1. Field of the Invention
The present invention relates to a surface electric potential sensor for detecting an electric potential on a surface of an object without contacting the surface. There are two types of surface electric potential sensors--one is a sector type which chops the lines of electric force by a grounded high conductive member, and the other is a vibrating-reed electrometer (VREM) type which periodically changes the capacitance between the surface to be measured and an electrode reed receiving the lines of electric force. The present invention relates particularly to the VREM type surface electric sensor.
2. Description of the Prior Art
There is a vibrating-reed type electrometer which uses a needle electrode mounted on the tip of one leg of a tuning fork driven by a piezoelectric vibrator as an electrode for receiving the line of electric force emitted from a surface of an object to be measured. Since a measuring probe mounting therein the needle electrode is relatively large in size and extremely expensive, this type of electrometer is usually used not for simple monitoring but for test (standard) measurement. Examples of this type are disclosed in U.S. Pat. No. 4,205,267 (Reference (1)) and Electrophotography, the Journal of the Society of Electrophotography of Japan, Vol. 8 (1983), No. 1, p. 52 (Reference (2)).
On the other hand, a simple surface electric potential sensor of the sector type is in the market. This sensor uses a metal tuning fork as an electric field chopper and receives the line of electric force by a stationary electrode. Since the sensor installs parts of a signal amplifier circuit on both surfaces of a printed circuit board (PCB) for holding the tuning fork, it is complicated in structure (difficult to be produced) and requires a large casing. The sensor is disclosed in NEC Technical Journal, Vol. 36 (1983), No. 1, p. 132 (Reference (3)).
As a VREM type sensor, such a sensor has been proposed that uses a leg of a metal tuning fork as an electrode for receiving the line of electric force. In this sensor, since the base of the tuning fork is made of an insulating material different from the material of the legs, it is difficult to integrally manufacture the base and legs in order to facilitate vibrations of the tuning fork which is the main element of the sensor. Further, it is not easy to match acoustic impedance between the base and legs of the tuning fork, so that the vibration efficiency of the tuning fork is low. Furthermore, since a piezoelectric vibrator necessary for driving the tuning fork is provided on only one of the two legs, the vibration efficiency cannot be made high. As a result, the sensitivity (S/N ratio) of the sensor is deteriorated because the reduction of the tuning fork vibration efficiency, providing the vibration frequency be constant, causes a decrease in vibration amplitude of the tuning fork, which means that the range of capacitance changes between the electrode and the surface to be measured reduces. This sensor is disclosed in Japanese Laid-Open Patent Application 57-133360 (Reference (4)).